1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having high image quality, and a method for driving the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices display images by controlling arrangement of liquid crystals. The LCD devices have such advantages as lightweight, slim profile and low power consumption. Thus, the LCD devices are widely used in portable computers, office automation instruments, and so on.
FIG. 1 is a block diagram of an LCD device according to the related art.
Referring to FIG. 1, the LCD device includes a liquid crystal panel 2 on which an image is displayed, a gate driver 4 and a data driver 6 for driving the liquid crystal panel 2, a timing controller 10 for controlling the gate driver 4 and the data driver 6, a backlight unit 8 for supplying light to the liquid crystal panel 2, and a backlight driver 12 for driving the backlight unit 8.
The timing controller 10 rearranges image data supplied from a system (not shown) into red image data, green (G) image data, and blue (B) image data. The timing controller 10 generates a gate control signal and a data control signal using horizontal/vertical sync signals (Vsync, Hsync) supplied from the system (not shown). In addition, the timing controller 10 controls the backlight driver 12.
The data driver 6 supplies data signals to data lines according to the data control signal provided from the timing controller 10. The gate driver 4 sequentially supplies scan signals to gate lines according to the gate control signal provided from the timing controller 10.
The liquid crystal panel 2 includes two glass substrates. Liquid crystal is provided between the two substrates. In the liquid crystal panel 2, a plurality of pixels defined by a plurality of gate lines and a plurality of data lines are arranged in a matrix configuration. Each pixel has a thin film transistor (TFT).
The liquid crystal is driven in accordance with the image data. That is, the liquid crystal is driven by a potential difference between a common voltage and an analog data voltage corresponding to the image data. The potential difference determines an amount of light emitted from the backlight unit 8 and transmitted through the liquid crystal and a gray level. A liquid crystal driving voltage, which will be described below, means the potential difference between the common voltage and the analog data voltage corresponding to the image data.
FIG. 2A is a waveform illustrating a response time of liquid crystal.
Referring to FIGS. 1 and 2A, a liquid crystal driving voltage A changes from a low level to a high level, and a backlight driving voltage B maintains a constant DC voltage. The backlight driving voltage B is supplied from the backlight driver 12.
As the analog data voltage corresponding to the image data is supplied to the data line of the liquid crystal panel 2, the liquid crystal driving voltage A is applied to the liquid crystal and thus the liquid crystal responds to the liquid crystal driving voltage. In this case, a liquid crystal response characteristic C increases slowly from a low level to a high level. Therefore, the liquid crystal does not perfectly respond to the liquid crystal driving voltage A within one frame period.
The liquid crystal response characteristic C has a close relationship with a light transmission characteristic D. That is, the light transmission characteristic D of a backlight passing through the liquid crystal is mainly determined by the liquid crystal response characteristic C.
Because the liquid crystal does not respond perfectly within one frame period, the light transmission characteristic D cannot have the desired brightness. As a result, a motion blurring phenomenon is generated in a moving picture. Further, the contrast ratio is reduced and thus the display quality is degraded. To solve the slow response time of the LCD device, an over driving circuit (ODC) driving scheme has been proposed.
FIG. 2B is a waveform illustrating a response time of liquid crystal in an ODC driving scheme.
Referring to FIGS. 1 and 2B, a backlight driving voltage B′ maintains a constant DC voltage. The backlight driving voltage B′ is supplied from the backlight driver 12. A liquid crystal driving voltage A′ has a higher level than the liquid crystal driving voltage A of FIG. 2A.
As an analog data voltage corresponding to image data is supplied to a data line of the liquid crystal panel 2, the liquid crystal driving voltage A′ (higher than the liquid crystal driving voltage A of FIG. 2A) corresponding to a potential difference between the analog data voltage and the common voltage is applied to the liquid crystal, and thus the liquid crystal responds to the liquid crystal driving voltage A′. In this case, a liquid crystal response characteristic C′ is improved compared to the liquid crystal response characteristic C because the liquid crystal responds more quickly to the liquid crystal driving voltage A′, which is higher than the liquid crystal voltage A of FIG. 2A. Because a light transmission characteristic D′ is mainly determined by the liquid crystal response characteristic C′, the light transmission characteristic D′ is also improved as the liquid crystal response characteristic C′ is improved. Therefore, a desired brightness can be quickly obtained within one frame period. Accordingly, the ODC driving scheme can minimize the motion blurring problem by improving the response time of the liquid crystal and improve the contrast ratio of the LCD device.
However, the ODC driving scheme alone cannot perfectly solve the motion blurring problem. To further minimize the motion blurring phenomenon, a scan backlight driving scheme has been proposed.
FIG. 2C is a waveform illustrating a response time of liquid crystal according to an ODC driving scheme and a scan backlight driving scheme.
Referring to FIGS. 1 and 2C, a liquid crystal driving voltage A″ has a higher level than the liquid crystal driving voltage A of FIG. 2A. That is, the liquid crystal driving voltage A″ has a higher level than the liquid crystal driving voltage A of FIG. 2A during the first frame. However, the liquid crystal driving voltage A″ has a level identical to the liquid crystal driving voltage A of FIG. 2A after the first frame. In addition, a backlight driving voltage B″ does not remain constant and increases from a low level to a high level during the first frame and it then decreases to a low level at the end of the first frame. This procedure can be repeated throughout the frames.
As an analog data voltage corresponding to image data is supplied to a data line of the liquid crystal panel 2, the liquid crystal driving voltage A″ is applied to the liquid crystal, and therefore the liquid crystal responds to the liquid crystal driving voltage A″. In this case, a liquid crystal response characteristic C″ is improved because the liquid crystal responds more quickly to the liquid crystal driving voltage A″, which is higher than the liquid crystal voltage A of FIG. 2A during the first frame.
After the liquid crystal driving voltage A″ is applied, the backlight driving voltage B″ maintains a low level during an initial period of time. Accordingly, even though the liquid crystal responds quickly to the liquid crystal driving voltage A″ applied thereto, no light is emitted from the backlight unit 8. Thus, no light passes through the liquid crystal panel 2. As a result, a light transmission characteristic D″ is different from the light transmission characteristic D′ of FIG. 2B. That is, because the backlight driving voltage B′ of FIG. 2B is a DC voltage with a constant level, the light transmission characteristic D′ slowly increases from a zero level. On the contrary, because the backlight driving voltage B″ of FIG. 2C has both a low level and a high level in every frame, the light transmission characteristic D″ increases from a low level to a high level when the backlight driving voltage B″ has a high level. Because the liquid crystal has already been driven when the backlight driving voltage B″ increases to a high level, the light transmission characteristic D″ increases immediately from a low level to a high level.
When the backlight driving voltage B″ increases from a low level to a high level, light emitted from the backlight unit 8 passes through the liquid crystal 2 in a state in which the liquid crystal responds quickly, so that a desired uniformity can be achieved. Likewise, no light passes through the liquid crystal panel 2 during the initial period of time in the frame. After the initial period of time, light passes through the liquid crystal panel 2. In this way, the motion burring phenomenon can be further minimized.
Although the motion blurring phenomenon can be minimized by the ODC driving scheme, the scan backlight driving scheme and the combined method thereof, there is a limitation in improving the response time of the liquid crystal. Due to this limitation, it is difficult and takes a long time to obtain a desired brightness.